How many drug injectors are there in New Haven? Answers from AIDS data

Abstract

The number of drug injectors in a given community is an important quantity to determine, yet estimating this quantity has proven very difficult. This is not surprising given the nature of drug injecting behavior and its illicit status; direct enumeration of the population of drug injectors is not possible. This paper details an indirect approach grounded in the epidemiology of HIV infection, a disease inextricably linked to drug injecting behavior. The method consists of two parts. First, one determines from AIDS incidence data the aggregate rate of new HIV infections among drug injectors. Second, one determines a per capita HIV incidence measure among drug injectors (either empirically via a cohort study, or analytically via a mathematical model). Division of the first quantity by the second yields an estimate of the number of drug injectors in the community. Natural measures of variablility follow from this approach, enabling the construction of confidence intervals. When applied to New Haven, Connecticut (where the necessary data are available, in part owing to the evaluation of that city's legal needle exchange program), the technique estimates roughly 1,910 to 2,660 drug injectors, or between 1.5% and 2.1% of New Haven's total population.